This disclosure relates generally to the field of digital image capture and processing. More particularly, this disclosure relates to techniques for improved autofocus operations in a digital image capture device. A camera's autofocus (AF) system automatically adjusts the camera lens to obtain focus on a subject. In general, AF systems use contrast sensors within the camera (passive AF) or by emitting a signal to illuminate or estimate the distance to a subject (active AF). Passive AF may employ either contrast detection or phase detection methods, but both rely on contrast for achieving autofocus.
Some digital cameras utilize autofocus sensors that are different from the camera's image sensor. This may, for example, occur in single-function digital cameras. In small multifunction devices such as mobile phones, tablet computers, personal digital assistants and portable music/video players, however, separate image and autofocus sensors are often not used. In devices such as these, autofocus operations can involve adjusting the position of the device's lens (or lens assembly) at a specified number of locations; evaluating the focus (e.g., contrast) between corresponding points in successive images (maximum contrast is assumed to correspond to maximum sharpness or “best” focus).
Multifunction devices typically evaluate contrast/focus at a fixed number of points in an image and, when the lens is moved to obtain successive images, a fixed number of lens positions. As such, these devices can provide autofocus operations at a fixed rate and with a fixed resolution. Thus, it would be beneficial to provide a mechanism to dynamically improve the speed or resolution of a digital image capture device.